Ask the Doctors

Dear Doctor: Can marijuana use really increase the risk of high blood pressure, as a recent study suggests? I thought marijuana was supposed to make you relax.

Dear Reader: If you'll recall, there was a time when smoking tobacco was similarly touted as a healthy way to relax. Only after years of study and national educational campaigns did the public began to realize the detriments of tobacco. Today, in the desire to legalize marijuana, proponents have focused on its potential health benefits. That doesn't mean it has no negative effects.

In fact, smoking marijuana leads to an increase in heart rate, increased contraction of the heart and a small increase in blood pressure. The 2017 European Journal of Preventive Cardiology study you mentioned attempted to quantify these negative effects.

The data used in the study come from a national survey in 2005 in which people over the age of 20 were asked: "Have you ever used marijuana or hashish?" Participants who answered "yes" were termed "marijuana users." The authors also asked the year that people first used marijuana, from which they calculated total years of use. The authors then looked at mortality data from 1991 to 2011.

In this study, 56.5 percent of the 1,213 eligible participants were qualified as marijuana users. Note that 63 percent of these "users" also had a history of smoking tobacco. The overall death rate of marijuana users was 29.7 percent, while in non-users the rate was 26.2 percent. The authors found that the death rate from high blood pressure was 4.3 percent higher in the users of marijuana, but they didn't find a difference in death rates from heart attacks or strokes.

But let's look at the major problems with this study. First, the definition of "marijuana user" was problematic. People who used marijuana once in their lifetime were quantified as marijuana users. The data didn't differentiate between someone who had used marijuana every day for the last 20 years and someone who used it once in college. Next, the number of participants was relatively small, which affects the conclusiveness of the findings. For example, in this study, cigarette smokers had less risk of dying from a heart attack than did non-smokers. That runs counter to large trials showing the opposite. Lastly, many of the marijuana smokers also smoked cigarettes -- so if the data about cigarette smokers were inaccurate, as the suspect finding suggests, there may be further inaccuracies in the data.

However, the study does highlight that marijuana users had a higher rate of high blood pressure. This was also found in a 2016 study showing a mild elevation of blood pressure in marijuana users. Among people who used marijuana one to six days per month, the systolic blood pressure was elevated by 1.3 points on average, and among those who used 21 to 30 days per month, the systolic blood pressure was elevated by 2.6 points on average.

All these data point to how poorly we understand the health consequences of marijuana use. For example, we simply don't know whether marijuana smoking has the same negative vascular effects as tobacco smoking. With less fear of incarceration over marijuana use, perhaps more people will participate in larger studies to assess marijuana's effect upon the cardiovascular system.